Size dependence of nanostructures: Impact of bond order deficiency

This report deals with the mechanism behind the unusual behavior of nanostructures in mechanical strength, thermal stability, acoustics (lattice dynamics), photonics, electronics, magnetism, dielectrics, and chemical reactivity and its indication for designing and fabricating nanostructured materials with desired functions. A bond-order-length-strength (BOLS) correlation mechanism has been initiated and intensively verified, which has enabled the tunability of a variety of properties of a nanosolid to be universally reconciled to the effect of bond order deficiency of atoms at sites surrounding defects or near the surface edges of the nanosolid. The BOLS correlation indicates that atomic coordination imperfection causes the remaining bonds of the under-coordinated atom to contract spontaneously associated with bond strength gain and the intraatomic trapping potential well depression. Consequently, localized densification of charge, energy and mass occurs to the surface skin, which modify the atomic coherency (the product of bond number and the single bond energy), electroaffinity (separation between the vacuum level and the conduction band edge), work function, and the Hamiltonian of the nanosolid. Therefore, any detectable quantity can be functionalized depending on the atomic coherency, electroaffinity, work function, Hamiltonian or their combinations. For instances, the perturbed Hamiltonian determines the entire band structure such as the band-gap expansion, core-level shift, Stokes shift (electron-phonon interaction), and dielectric suppression (electron polarization); The modified atomic coherency dictates the thermodynamic process of the solid such as self-assembly growth, atomic vibration, phase transition, diffusitivity, sinterbility, chemical reactivity, and thermal stability. The joint effect of atomic coherency and energy density dictates the mechanical strength (surface stress, surface energy, Young's modulus), and compressibility (extensibility, or ductility) of a nanosolid. Most strikingly, a combination of the new freedom of size and the original BOLS correlation has allowed us to gain quantitative information about the single energy levels of an isolated atom and the vibration frequency of an isolated dimer, and the bonding identities in the metallic monatomic chains and in the carbon nanotubes. A survey and analysis of the theoretical and experimental observations available to date demonstrated that the under-coordinated atoms in the surface skin of 2-3 atomic layers dictate the performance of nanostructures yet atoms of the interior remain as they are in the bulk counterpart. Further extension of the BOLS correlation and the associated approaches to atomic defects, impurities, liquid surfaces, junction interfaces, and amorphous states and to the temperature domain would be more challenging, fascinating, and rewarding. (C) 2006 Elsevier Ltd. All rights reserved.